Often, fasteners used to assemble performance critical components are tightened to a specified torque level to introduce a “pretension” in the fastener. For example, high tensile-strength steel bolts used to fasten components of military vehicles, aerospace vehicles, heavy machinery, and equipment for petrochemical operations frequently have required torque specifications. Torque is applied to the head of the fastener, which causes the fastener to stretch beyond a certain level of applied torque. This stretch results in pretension in the fastener which then holds the joint together. Overstressed bolts can lead to breakage whereas under-stressed bolts can lead to loosening of the fastener. Furthermore, an unequally stressed set of fasteners can result in gasket distortion and subsequent problems like leakage. Accurate and reliable torque wrenches help insure that fasteners are tightened to the proper torque specifications.
There are several types of torque wrenches that are routinely used to tighten fasteners to specified torque levels: mechanical and electronic torque wrenches. One of the more common mechanical-type torque wrenches, the clicker type mechanical torque wrench, makes an audible click to let the user know when a certain torque level has been achieved, and simultaneously provide a feeling of sudden torque release to the user. One example of a clicker torque wrench has a hollow tube in which a spring and pawl mechanism is housed. The pawl is forced against one end of a bar that is connected to a drive end. The bar and a drive head are pinned to the hollow tube and rotate as torque is applied. The pawl is released when the force applied by the bar increases beyond a preset torque level, the preset torque level being set by the spring acting on the pawl. When released, the bar hits the inside of the tube and produces a sound and a sudden torque release that is detectable by the user. Typically, the torque values are permanently marked on a drum type scale that is visible through a window near or on the handle, or marked on the tube itself.
Electronic torque wrenches utilize electronic circuitry for measuring and displaying torque values and typically have a keypad with multiple keys that are capable of a number of functions. A transducer sensor is mounted in the wrench handle and measures the shearing stress being applied to the transducer as the wrench is rotated. The transducer is electrically coupled back to a processor provided on or in the handle, which calculates the resulting torque based on the shearing stress being measured. One disadvantage of electronic torque wrenches is that they typically cover a narrow torque band that can be measured. Thus, multiple wrenches must be used to accurately cover a wide range of measurable torque.
The present invention recognizes and addresses the foregoing disadvantages, and others, of prior art constructions and methods.